1. Field of the Invention
This invention relates to a process for the preparation of Nickel base superalloys and articles made thereof.
2. Description of the Related Art
The performance of gas turbine engines is limited by the high temperature capability of turbine blades and nozzle guide vanes (NGV) in the engines. A hot combustion gas-air mixture containing highly corrosive ingredients is directed at high pressure and velocity against the NGV parts in such engines. This hot gas then expands through the turbine blades and imparts its kinetic energy to the rotating turbine blades. Gas turbine blades and vanes therefore operate in extremely hostile environment of high temperatures, high stresses, oxidation and hot corrosion and accordingly, the materials development for such a critical application has been quite challenging. The directionally solidified (DS) columnar grained alloy CM 247 LC has been successfully used in the prior art up to blade or vane metal temperature of about 1070xc2x0 C. Further improvement was possible after partial substitution of W and Cr from the CM 247 LC alloy composition by 3 wt. % of Re to result in the second generation columnar grained alloy CM 186 LC. About 160xc2x0 C. improvement over the temperature capability of CM 247 LC could be realised by the CM 186 LC alloy. This advantage is attributed to the presence of Re and a higher total refractory elements of 16 wt. % compared to that of 14.6 wt. % of CM 247 LC. Modern aero-engines with most advanced single crystal superalloys in their first stage turbine components however required far greater temperature capability than that of the CM 186 LC alloy for their subsequent stages of turbine parts which are often impractical to cast in single from.
Another disadvantage of the processes known in the prior art for preparing a hollow columnar grained component is the requirement of ceramic cores which leads to higher rejection of components.
Yet another disadvantage of the processes known in the art is the molten alloy and ceramic core reaction that leads to appreciable loss of key alloying elements which in turn deteriorates materials performance. Further disadvantage with the columnar grained materials of prior art is that a good fraction of Re and other refractory elements are not utilized for creep resistance as they are locked up in a coarse gamma prime phase, whose solutionization often leads to recrystallization and deleterious phase formation and deteriorates creep resistance.
The primary object of the present invention is to propose Ni-base superalloys and articles made therefrom with higher refractory element content beyond that of the CM 186 LC alloy in order to achieve superior creep resistance for applications such as gas turbine blades and vanes with adequate resistance to oxidation, hot-corrosion and deleterious phase formation.
Another object of the present invention is to propose Ni-base superalloys and articles made therefrom with excellent castability for advanced gas turbine blades and vanes in polycrystalline form having thin walled aero-foils, shrouded segments and intricate cooling channels.
A further object of the present invention is to propose a process for preparation of hollow columnar grained turbine components having complex cooling channels without employing ceramic cores and improving the yield of quality components.
Yet a further object of the present invention is to propose a process for preparation of hollow columnar grained materials which can be heat treated without causing any recrystallization to achieve improved balance of critical mechanical properties for advanced gas turbine engine application.
A still further object of the present invention is to propose columnar grained materials which can be conveniently brazed and given protective coating by existing manufacturing techniques during heat treatment.
Another object of this invention is to propose a columnar grained alloy composition out of the most advanced single crystal alloy chemistry and ensure thereby component castability as well as performance superiority over prior art materials.
These and other objects and advantages will be more clearly understood from the following detailed description, drawings and specific examples which are intended to be typical of, rather than in any way limiting on the scope of the present invention.
According to this invention there is provided a Nickel base super alloy comprising,
in weight %, 1.4 to 4.4. Cr, 3-8 Co, 5-7.5 W, 4.8-7.5 Re, 6-9 Ta, 4.8-6 Al, 0.1-0.5 Nb, 0.8-1.8 Hf, 0.05-0.1 C, 0.01-0.05 Y, 0.005-0.015 B the balance being Nickel.
In accordance with the present invention, the sum of the weight percentage of W+Re in the superalloy is about 12. The sum of the weight percentage of Al+Ta+Hf+Nb in the superalloy is from 13.6-15.6 and the total refractory element (W+Re+Hf+Nb) content is from about 20.2 to 22.8, the alloy being substantially free of S and V. The alloy preferably comprises a uniform distribution of Ni3Al type cubical xcex3 particles in a gamma (Y) matrix of Ni-base solid solution together with carbides.
The preferred range of the alloying elements which leads to producing further optimized properties comprises, by weight percent, from about 1.8-3.8 Cr, 4-6.4 Co, 5.2-6.8 W, 5.8-7.2 Re, 6-9 Ta, 5.2-5.9 Al, 0.8-1.6 Hf, 0.1-0.5 Nb, 0.06-0.1 C, 0.015-0.045 Y, 0.005-0.015B, balance essentially Ni, wherein the sum of W+Re is about 12, the sum of Al+Ta+Hf+Nb is from about 13.6 to 15.6 and the total refractory element content is between 20.2 and 22.5, the alloy being substantially free of S and V.
The most preferred range of composition by weight percent is essentially 1.8 to 3 Cr 4 to 6.4 Co, 5.2 to 6 W, 6 to 7.2 Re, 6.5 to 9 Ta, 5.2 to 5.5 Al, 0.8 to 1.6 Hf, 0.1 to 0.3 Nb, 0.07 to 0.015 to 0.04 Y, 0.01 to 0.015 B, balance being essentially Ni wherein the sum of W+Re is about 12 the sum of Al+Ta+Hf+Nb is in the range of 13.6 to 15.6 and the total refractory element (W+Re+Hf+Nb) content is in the range of 20.2 to 22.5, the alloy being substantially free of S and V.
Further according to the present invention there is provided a process for the preparation of Ni-based superalloy article comprising the steps of:
a) preparing an alloy charge by adding, in weight percent 1.4-4.4 Cr, 3-8 Co, 5-7.5 W, 4.8-7.5 Re, 6-9 Ta, 4.8-6 Al, 0.1-0.5 Nb, 0.8-1.8 Hf, 0.05-0.1 C, 0.01-0.05 Y, 0.005-0.015 B, the balance being Ni;
b) melting the alloy charge prepared by step (a):
c) pouring the said melt into a mould;
d) withdrawing the mould containing melt;
e) subjecting the melt to a step of freezing;
f) removing and cleaning each pair of cast longitudinal halves and subjecting them to solution heat treatment:
g) subjecting the brazed and solutionised components obtained by steps (f) to a step of quenching;
h) subjecting the above to a multistep ageing treatment.
Further in accordance with the present invention a Nickel base superalloy charge is prepared by adding, in weight %, of 1.4 to 4.4 Cr, 3-8 Co,5-7.5 W, 4.8-7.5 Re, 6-9 Ta, 4.8-6 Al, 0.1-0.5 Nb, 0.8-1.8 Hf, 0.05-0.1 C, 0.01-0.05 Y, 0.005-0.015 B, the balance being Nickel, with W+Re being 12, Al+Ta+Hf+Nb being 13.6-15.6 and total refractory elements W+Re+Ta+Hf+Nb being 20.2-22.8 in a vaccum induction melting furnace at a temperature for example 1500xc2x0 C. The melt is then poured into a mould. The mould has several cavities in pairs of two longitudinal halves of each component, each pair being connected at the bottom to a central sprue through bottom runners.
The mould is preheated to a temperature of, for example 1500xc2x0 C. in a vacuum melting and casting furnace and then the melt is poured into the mould.
The mould containing melt is withdrawn from the mould-heater of the vacuum melting and casting furnace, across a temperature gradient so as to induce directional freezing of the melt, either by radiactive heat transfer to the water-cooled furnace chamber or by conduction into a low melting pool of liquid metal such as aluminum.
Each pair of cast longitudinal halves is removed and cleaned and then subjected to solution heat treatment in a stepped manner between about 1275xc2x0 C. to 1295xc2x0 C. for about 24 hours, the peak soaking temperature being at least 10xc2x0 C. below the alloy incipient melting temperature. During this solution heat treatment, the two cast halves are held against each other long their matching plane of slice incorporating desirable braze-filler material so that a complete single crystal component having intricate cooling channels would form by brazing of the two halves simultaneously along with solutionisation. The solution heat treatment dissolves the irregular xcex3-particles and most or all of the eutectic-xcex3 into the xcex3-matrix depending upon the casting geometry and casting conditions of the article.
The brazed and solutionised components are subjected to argon gas fan quenching followed by a multi-step ageing treatment in order to precipitate the xcex3-particles within the xcex3-matrix. In the first step the components are heated to about 1170xc2x0 C. and held there for about 4 hours. In the next ageing-step, the components are given simulated coating treatment of holding at about 1140xc2x0 C. for about 4 hours. Next, the components are aged at about 870xc2x0 C. for about 20 hours and finally cooled down to the ambient temperature. The ageing treatment is not limited to this preferred heat treatment sequence, but instead may be accomplished by any acceptable manner which provides the desired volume fraction of xcex3xe2x80x2-particles in a somewhat regular array, cubical shape and uniform size around 0.5 xcexcm.
Based upon an evaluation of alloys prepared in accordance with the invention, preferred and most preferred ranges of alloying elements have been determined. The evaluation procedures are described in connection with the examples to be presented subsequently. The preferred range of composition in wt % consists essentially of from about 1.8 to 3.8 Cr, 4 to 6.4 Co, 5.2 to 6.8 W, 5.8 to 7.2 Re, 6 to 9 Ta, 5.2 to 5.9 Al, 0.8 to 1.6 Hl, 0.1 to 0.5 Nb, 0.06 to 0.1C, 0.015 to 0.045 Y, 0.005 to 0.015 B, balance Ni, wherein the sum of W+Re is about 12, the sum of Al+Ta+Hf+Nb is from about 13.6 to 15.6 and the total refractory elements (W+Re+Hf+Nb) is between 20.2 and 22.5, the article being substantially free from S and V. It is found that this preferred range provides improved combination of critical properties relative to what have been obtained in columnar grained superalloy articles of the prior art.
The most preferred range of composition by weight percent is essentially 1.8 to 3 Cr, 5 to 6.4 Co, 5.2 to 6 W, 6 to 7.2 Re, 6.5 to 9 Ta, 5.2 to 5.5 Al, 0.8 to 1.6 Hf, 0.1 to 3 Nb, 0.07 to 0.1C, 0.015 to 0.04 Y, 0.01 to 0.015 B, balance being essentially Ni, wherein the sum of W+Re is about 12, the sum of Al+Ta+Hf+Nb is in the range of 13.6 to 15.6 and the total refractory element (W+Re+Hf+Nb) content is in the range of 20.2 to 22.5, the alloy being substantially free of S and V.
The articles of the present invention are particularly suitable for use as turbine blades and vanes owing to their excellent high temperature mechanical and environmental properties, as well as their alloy phase stability. Other features and advantages of the present invention will become apparent from more detailed descriptions, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.